Answer:
B: I searched the yard, but I didn't see my brother there.
Step-by-step explanation:
A is incorrect because of "they're" which would need to be "their" (they're stands for they are).
C is incorrect because of "there" which would need to be "their" (there points to something).
D is incorrect because of "their" which would need to be "they're" (their stands for something they own).